Method of reducing corrosion



United States Patent 3,249,524 METHOD OF REDUCING CORROSION David B. Sheldahl, Griffith, Ind., assignor to Sinclair Research, Inc., Wilmington, Del., a corporation of Delaware No Drawing. Filed Nov. 1, 1961, Ser. No. 149,192 7 Claims. (Cl. 204-148) The present invention relates to the corrosion of ferrous metals. More specifically, the present invention relates to a method of reducing the corrosion of the surfaces of ferrous metals caused by contact with aqueous ammonium nitrate solutions.

There is a well recognized corrosion problem in industries concerned with the manufacture, storage, transportation and handling of ammoniacal-ammonium nitrate solutions. In the handling of such solutions it is convenient to transport and store them in ferrous containers such as drums, tanks and pipelines. However, in view of the corrosive nature of ammoniacal-ammonium nitrate solutions against ferrous metals, many manufacturers now use storage and transportation facilities constructed of' aluminum. Aluminum is used because its oxide film renders the metal inert to attack by the ammoniacal salt solution. This remedy, however, is a costly one. Corrosion inhibitors of one type or another also have been suggested and attempted with varying degrees of limited success. For example, trivalent arsenic oxide, an arsenite such as sodium, potassium or ammonium arsenite, and sulfides of trivalent arsenic are commonly employed in aqueous ammoniacal ammonium nitrate solutions alone or in combination with other inhibitors but their use is seriously limited by their low solubility in such solutions, particularly in those solutions wherein the free ammonia/ water weight ratio exceeds about 1.5.

One effective method for remedying the problem has been to deactivate the ferrous metal, for instance, by passivating the metal surface. Passitivity is a property exhibited by some metals whereby they become inactive toward certain chemical reagents. When a piece of reactive metal is made passive, its position in the electrochemical series is changed so that it is cathodic to a piece of the same metal which is in the active condition. Passivation of ferrous metals employed in a corrosive environment, is generally accomplished by contacting the metal'with an oxidizing agent. The oxidizing agent reacts with the ferrous metal forming a thin adherent gamma- Fe O film on its surface. This protective film shields the ferrous metal from its environment and virtually no corrosion occurs.

It is known, as described by Charles K. Lawrence et al.

in U.S. Patent No. 2,366,796 that corrosion of ferrous surfaces by ammoniacal solutions can be reduced or prevented by making the ferrous metal in contact with the solution the anode in 'an electric circuit which includes an inert cathode in the solution and imposing a direct current of a particular voltage from an outside source through the electric circuit. .In cases of ferrous containers of large size such as tank cars or storage tanks, however, the patente'es disclose that the application of a current to the electrodes is insufiicient to prevent corrosion of the container unless a cathode is employed having a surface area approaching that of the container. Inasmuch as use of cathodes of this size in these containers is impractical, the patentees disclose inhibiting such containers against corrosion by first passivating the containers with an oxidizing agent and then maintaining this passivity by making the passivated ferrous container the anode, dipping an inert cathode in the ammoniacal solution and applying from an outside source of direct current of at least 1.25 volts and not above 2.2 volts.

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It has now been discovered that if carbon is connected to the ferrous metal by an electrically conductive path a ferrous metal-carbon galvanic couple is established which generates sufficient current in situ, that is without the use of outside current, to accomplish passivation of the ferrous metal and thereby corrosion is reduced by anodical protection. Furthermore, once passivation has been achieved, enough current is available from the ferrous metal-carbon galvanic couple to maintain the passivity. The carbon is in electrical contact with the ferrous surfaces to be protected and with the aqueous ammonium nitrate solution. The electrically conductive path between the ferrous surface and the carbon can be provided conveniently by attaching a carbon electrode directly to the ferrous metal surface which contacts the nitrate solution, the attachment being at a point where the carbon also contacts the solution, for instance, the carbon may be attached to the inside, bottom of a steel tank. Alternatively, the carbon can be supplied as an example, by forming a relatively thin coating containing minute carbon particles on the metal surface contacting the ammonium nitrate solution as disclosed and claimed in application Serial No. 149,186 (now US. Patent No. 3,078,- 991), filed concurrently herewith in the names of Franklin M. Watkins and David B. Sheldahl and hereby incorporated by reference.

The current density created by the ferrous metal-carbon galvanic couple is dependent on the surface area ratio of the active ferrous metal to carbon in contact with the solution. The active ferrous metal is the non-passivated ferrous surface in contact with the solution and thus the rate of addition of the ammonium nitrate solution to the container may affect the extent of corrosion protection obtained. Accordingly, a surface area ratio of ferrous metal to carbon and/or a rate of addition should be selected to produce a current density sufficient to passivate the ferrous surface. For example, the current density can be increased by increasing the area of the carbon or by slowing the rate of filling the container. When the ratio of surface area of the ferrous metal to carbon is about 1:1 to 15:1, for instance, it has been found that sufficient current density is generated even to passivate ferrous metal containers to which the ammonium nitrate solutions are rapidly added. On the other hand, much greater ferrousmetal to carbon surface areas, say of about 20:1 up to 200 or even more:1, can be used when the ammonium nitrate solution contacts the metal slowly. A very practical surface area ratio is about 50 to :1.

Ordinarily, a current density. (Working) in the range of u at least about 0.1 to about 1.5 or more ma./cm. of active metal surface is found sufficient for passivation of the ferrous metal and the voltage resulting may be about 0.7 to .75 volt.

' Thus, in accordance with the present invention corrosion of the surfaces of a ferrous container due to contact with the ammonium nitrate solutions can be prevented or reduced by providing the container with an elemental carbon electrode and metallically and electrically connecting the carbon electrode with the ferrous metal surface of the container to establish a ferrous metal-carbon galvanic coupie. It is preferred that the carbon electrode be directly connected to the bottom of the container particularly in the case of containers of large size such as storage tanks, tank cars and the like so that the current density generated by the couple is sufficiently great to passivate the ferrous metal as the solution contacts it. This construction maintains the carbon electrode in contact with the nitrate solution in the vessel. It has been found that by connecting the carbon cathode, preferably a plurality of carbon cathodes, to the bottom of a large container such as a tank car so that on addition of the nitrate solution it first con- 3 tacts the carbon cathode, i.e. does not contact the ferrous metal before contacting the carbon, the most effective passivation is accomplished because the current density required is obtained at initial contact of the nitrate solution which current density is maintained while filling the container.

The ammonium nitrate solutions may vary considerably in composition. Although my system protects vessels containing aqueous ammonium nitrate solutions, greater need and utility lies in protecting vessels employed to handle ammoniacal aqueous ammonium nitrate solutions. Generally representative of such solutions encountered in industry and which give rise to the corrosion problems discussed hereinbefore, are those having approximately about 1 to 80 or more percent ammonium nitrate, usually at least about 40 percent preferably about 60 to 70 percent; about 5 to 35 percent free ammonia, preferably about to percent; and the substantial balance being water, for instance, about 10 to 25 percent water. These percentages are by weight.

The ammonium nitrate solutions may contain additives well known to the art as corrosion inhibitors in these solutions. Examples of these inhibitors are trivalent arsenic compounds, for example, arsenic trioxide; an arsenite such as sodium, potassium and ammonium arsenites and sulfides of trivalent arsenic; compounds which contain divalent sulfur linked to an atom of carbon with the remaining valences of the carbon atom linking the carbon atom to nitrogen as, for instance, carbon disulfide, thiocyanates, thiocarboxylic acids, thioamides, etc. (see US. Patent No. 2,220,059 to Herman A. Beekhuis et a1.) and organic compounds having an SH and an OH group, for instance, as disclosed in US. Patent No. 2,613,131 to Marion D. Barnes et al. In fact the presence of these additives, particularly the arsenites, may enhance the passivation even in amounts for smaller than taught as effective by the prior art. The presence of compounds which provide the ammoniacal solution with copper and carbonate ion, for instance, basic cupric carbonate, have also been found to enhance the passivation. Not only is the presence of these additives of advantage in enhancing passivation but once passivation has been accomplished.

they act to further insure protection.

A particular effective inhibitor additive is the combination of trivalent arsenic compound, a soluble copper compound and carbonate ions, as disclosed in application Serial No. 5,637 in the names of Paul Shapiro, David B. Sheldahl and Lawrence V. Collings, filed February 1, 1960, now US. Patent No. 3,096,169. The soluble copper compound can be, for instance, the inorganic compounds such as cupric carbonates, hydroxides, sulfates, nitrates, etc. Of the many carbonate ion-producing compounds, the more particularly suitable are the inorganic compounds, for instance, alkali metal and ammonium carbonates. Preferably, the copper and carbonate components are provided by a single compound such as basic copper carbonate. The concentrations of the copper and carbonate components can vary considerably but are sulficient to give significant protection against corrosion. Generally, the concentration of the copper component is at least about .01 g./100 ml. of ammoniacal solution. The maximum amount of the copper compound is controlled by, economic feasibility and is generally not greater than about 0.2 g./100 ml. of ammoniacal salt solution. The preferred concentration is about .05 to .15 g./ 100 ml. of ammoniacal solution. The amount of carbonate compound employed is usually that sufficient to provide a carbonate ion concentration of at least about .005, generally about .02 to .1 g./100 ml. of ammoniacal solution. When basic cupric carbonate is employed, a concentration of about .01 to 0.2 g./100 ml. of ammoniacal solution, preferably about .05 to .15 is usually sufficient.

It has been noted that the corrosion by ammoniacal solutions is sometimes intense in the vapor zone, i.e. the portion of the vessel containing the ammoniacal solution which is in contact with vapors of the solution. Although the method of the present invention and corrosion inhibitors such as trivalent arsenic compounds or combinations of trivalent arsenic compounds with other inhibitors provide corrosion protection to the portion of the vessel in contact with the ammoniacal solutions, adequate protection is not always provided the portion in contact with vapor. This problem can be easily remedied by the addition to the solution of vapor phase inhibitors such as urea, NH NO etc. We have also found that the addition of NO -producing compounds such as an alkali metal nitrite to the ammoniacal solution containing the soluble copper and carbonate components very effectively reduces vapor phase corrosion and this may be due to the formation of a copper-ammonium-NO complex. The vapor phase inhibitor is generally present in an amount suflicient to provide adequate corrosion protection and conveniently is about .05 to .15 g./ ml. of ammoniacal solution.

The following examples are included to further illustrate the invention.

Example I As aforementioned when a piece of active metal is made passive, its position in the electrochemical series is changed so that it is more cathodic to a piece of the same metal which is in the active condition. Since the formation of passive films produces, a change in the electrical characteristics of a ferrous metal such as steel, i.e. makes the metal more electropositive, the. phenomenon can be effectively studied by observing changes in the single electrode potentialof the metal. A series of simple electrolytic cells were set up to achieve this end.

' A steel rod was first activated (i.e. all surface films were removed) by exposure to 15% HCl at F. until hydrogen bubbles were observed. The rod was then washed in deionized Water and placed in an electrolytic test cell filled with an ammoniacal solution consisting of 66.8% NH NO 16.6% NH and 16.6% .H O. The electrolytic test cell was a'large mouth 8-ounce glass jar having a salt bridge comprising a glass tube with agaragar solution saturated with KCl connected to a calomel cell immersed in saturated KCl. The calomel electrode probe and the activated rod were connected by leads to a Sheppard potentiometer by which potential measurements were obtained. A carbon electrode was immersed in the ammoniacal solutions and it as well as the steel specimen were connected to a Sheppard potentiometer for measuring potentials and currents derived therefrom.

The first series of tests was run in the solution containing no inhibitors. The single electrode potential of mild steel to calomel in this ammoniacal solution was found to be 0.760 volt. For carbon it was found to be 0.02 volt. The carbon and steel electrodes were electrically connected before the test solution was first addedvso that current flowed as soon as solution was added. The ratios of steel to carbon areas were the only variables. In all tests except R-200 the test solution was added rapidly. The results were as follows:

Run No. Area ratio of steel Passive Potentul tocaroon volts 1 calomel; above often to vary between -0.200 and 0.400 volt, the variation being due to surface conditions of the test specimen, small 'diflerences in steel composition, etc.) Test run R-ZOO demonstrates that much greater steel to carbon used as well as smaller quantities of trivalent arsenic if slower test solution rates had been used.

Example IV surface ratios can be used when the solution contacts the 5 steel very slowly. The test specimen was 6.6 cm. long, Tests were run as i P examljles but small and when the test solution was added slowly, three minutes amounts of both arsenlc tr1X1de and baslq PP s elapsed before the entire test specimen was completely bfmate were added to the ammonlasal solutlons immersed. The solution rose in the test cells at the rate was also added as a Vapor P corroslol'l f 0 036 o w the Solution was added 10 hibltor. The ratio of steel to carbon varied from 20:1 rapidly the test specimen was covered in three seconds. F 20011- In h cast? the test solutlon was added I h case h li i rose at 21 d idly. For comparison -a test was conducted on the solu- E l H tion containing no additives and employing a ratio of steel xamp e Y to carbon of 20:1. The results were as follows: Tests were conducted as in Example I with the same The results show that when both trivalent arsenic and Run Percent NaNO AS203 Ratio of steel Passive Potential No. CuCOzrCIKOHh to carbon volts 0.004 0.004" to1 Yes--." 0.310 0.0os -0.340 0.0s5. 0 .370 0.085-.-. 0. 340 0.085 -0.350 0.028-- -0.310 0.028. -0. 720 0.085-

0. 320 0.085-.-- 0.290 None 0. 760

ammoniacal solution but containing various amounts of basic copper carbonate. The ratio of steel to carbon was 20:1 and the test solution in each case was added rapidly. The results are as follows:

Run No. Percent Passive Potential OuC O3-Ou(OH)z volts The data demonstrate that when copper is present even copper are present in ammoniacal solutions passivity can be accomplished at steel to carbon area ratios up to as high as 100:1 even at rapid addition rates.

Example V Percent CuCOa-Cu(OH)2 Percent N EN 0 2 Ratio of steel Passive to carbon Carbon cathode Percent A520 3 in very small amounts, passivity is obtained at a steeltcarbon area ratio of-20:1. No doubt much greater steel to carbon area ratios could be used as well as small quantities of copper and passivity still obtained if slower test solution addition rates had been used.

Example III Tests were conducted as in Example II but with a small amount of AS203 instead of basic copper carbonate added to the ammoniacal solution as the inhibitor. The results are as follows:

1 Added as a vapor phase corrosion inhibitor.

The results show that the presence of a very small amount of trivalent arsenic provides passivity at a steel: carbon area ratio of 20:1. As in the case of coppenno doubt much greater carbon to steel area ratios could be arsenic trioxide and basic copper carbonate alone or together have difiiculty in passivating steel when ammoniacal solutions are employed, use of the steel:carbon galvanic couple with small amounts of the inhibitors in each case readily accomplishes passivity.

Example VI In accordance with the general method of Example I passive films were induced on steel test specimens in the ammoniacal solutions of Example I containing small amounts of arsenic alone, arsenic in conjuction with ammonium thiocyanate and the latter two additives in combination with basic copper carbonate. The steel to carbon area ratio of the galvanic couple employed in the passivation was 20: 1.

The stability of the film toward electrolytic destruction was then determined by the copper wire test or a more rugged electrical test. The copper wire test comprises exposing a No. 12 copper wire two inches long in the ammoniacal solution electrically connecting it to the steel test specimen. As soon as the potential falls below the Flade potential (the potential below which passivity cannot exist), the passive film has been destroyed. Since some passive films are too tough to reduce by the copper wire test the more rugged electrical test of stability was devised. In the latter test the passive film was subjected to a current from an outside source of 0.12 ma./cm. This current will cathodically reduce many passive films 8.. vanic couple was also present. Several tests were run using different ratios of steel to carbon. Solutions tested also contained different concentrations of arsenic. The current from an outside source and that generated by the carbon-steel couple were measured. 0.125% basic copin only a few minutes which the copper wire test (cop- 5 per carbonate was used in all soltuions. The results were as follows:

Current, Ina/cm. Ru Percent Percent Ratio, carbon No. NaNO; AS203 to steel Outside Steel-carbon Net cathodic source couple reduction R-l09--. 0. 064 0. 064 20 t 1 0. 12 0. 07 1 0. 050 Rl34 0. 064 0. 12 0. 036 Z 0 084 R-l l i--. 0. 043 0. 12 0. 052 2 0. 068 R 149... 0. 043 0. 12 0. 044 3 0. 070 R120 0. 028 0. 12 0. 064 4 0. 056 R-152.-- 0. 028 0. 028 30 to 1 0. 12 0. 036 0. 084

1 The test rod was not activated in 1 hour. 2 The test rod Was activated in 30 minutes. 3 The test rod was activated in minutes. 4 The test rod was activated in 10 minutes. 5 The test rod was activated in 5 minutes.

per-steel couple) will not reduce inmore than 24 hours.

The results of the tests are as follows:

The .data demonstrate that some of the current used for cathodic reduction can be negated by currents created 1 Copper wire activated steel test rod in 1 minute. 2 Copper wire activated steel test rod instantly. 3 Cathodic current of 0.12 mat/cm. activated steel test rod instantly. 4 Steel test rod became active spontaneously after 12 minutes. 5 Steel test rod became active spontaneously after minutes. 6 One-quarter of the rod was in the vapor zone throughout the entire test. 7 Cathodic current of 0.12 ma./cm. activated steel test rod in 2 minutes.

Note that all the passive films formed in the absence of copper were activated very easily. The passive film formed in test R-168 in the presence of copper required the use of the tougher film stability test to destroy passivity.

Example VII below:

Run Percent CuCOa. Percent Carbon Passive Potential N o. C11(OH)2 A5 03 cathode volts Rl71 0.031 0.021 Yes. Yes 0. 300 R-172- 0.031 0.021 No N0 0. 750

The data show that passivity is achieved with the carbon-steel galvanic couple in the range'of solubility that can be expected in the high ammonium-10w water ammonium nitrate solutions.

Example VIII Passive film were induced on steel test specimens in ammoniacal soltuions containing copper and arsenic in accordance with the general method of Example I. These passive films were then cathodically reduced by the more rugged electrical test described in Example VI (that is, by contact with a current from an outside source withv 0.1 ma./cm. current density) while a carbon-steel galby carbon-steel galvanic couples. In practice, it would therefore, be expected that any current generated by a break in the passive film could be negated immediately by the galvanic current generated by the carbon-steel couple thereby preventing breakdown of the passive film.

I claim:

1. A method for continuously reducing corrosion of the surfaces of a non-passivated ferrous metal in contact with an aqueous ammonium nitrate solution which comprises providing carbon in contact with said solution and an electrically conductive path between said carbon and said ferrous metal to create a ferrous metal-carbon galvanic couple generating sufficient current to passivate said ferrous metal surface and reduce corrosion thereof.

2. The method of claim-1 wherein the aqueous ammonium nitrate is an aqueous ammoniacal nitrate solution.

3. The method of claim 2 wherein created by the ferrous metal-carbon galvanic couple is in the range of approximately 0.1 to 1.5 ma./cm.

4. The method of claim 3 wherein the ammoniacal solution contains about 535% ammonia, about 40 to 70% ammonium nitrate and about 5 to 25% water.

5. The method of claim 4 wherein the ammoniacal solution contains corrosion inhibiting amounts of an ion selected from the group consisting of copper and trivalent arsenic.

6. The method of claim 4 wherein the non-passivated ferrous metal to carbon surface area ratio iswithin the range of about 50 to 200: 1.

7. The method of continuously preventing corrosion in non-passivated ferrous metal containers by aqueous ammoniacal ammonium-nitrate solutions which comprises connecting carbon to the lower portion of said container to provide a metallic electrically conductive path between said carbon and the non-passivated ferrous metal surface the current density 9 10 of said container, introducing into said container an aque- 2,576,680 11/1951 Guitton 204147 011s ammoniacal ammonium nitrate solution to generate 2,874,105 2/1959 Young 204-148 a current density in the range of approximately 0.1 to 3,010,886 11/ 1961 Chappell 204-149 rliurlajcgr to giisrigloafte said ferrous metal surface and 5 FOREIGN PATENTS 6 CO 1,097,749 2/1955 France.

References Cited by the Examiner OTHER REFERENCES UNITED STATES PA EN S Edeleanu: Metallurgia, September 1954, pages 113- 952,077 3/1910 Lutz 204-148 10 1,974,435 9/1934 Schulz et a1. 204148 2,366,796 1/ 1945 Lawrence et a1 204-147 JOHN C MURRAY TILLMAN, WINSTON 2,377,792 6/1945 Lawrence et a1 204-147 DOUGLAS, Exammers- 

1. A METHOD FOR CONTINUOUSLY REDUCING CORROSION OF THE SURFACES OF A NON-PASSIVATED FERROUS METAL IN CONTACT WITH AN AQUEOUS AMMONIUM NITRATE SOLUTION WHICH COMPRISES PROVIDING CARBON IN CONTACT WITH SAID SOLUTION AND AN ELECTRICALLY CONDUCTIVE PATH BETWEEN SAID CARBON AND SAID FERROUS METAL TO CREATE A FERROUS METAL-CARBON GALVANIC COUPLE GENERATING SUFFICIENT CURRENT TO PASSIVATE SAID FERROUS METAL SURFACE AND REDUCE CORROSION THEREOF. 